It will be found, on examining Appendix II., that in European stations by far the larger number of lamps are maintained from installations employing the Edison system; the Ferranti plan of using transformers comes next, closely followed by Goulard and Zippernowsky; the distribution with secondary batteries follows, and the high-tension multiple series comes last.

The Edison system has frequently been discussed, in connection with small installations, but in magnitude the stations in Berlin and in Milan exceed anything that has been started here with continuous current.

Before describing the central electric light station at the former city, it may be well to recall to mind that the Edison plan is the combination of a number of machines which pump electricity into a network of feeders, mains, and conductors, the lamps being placed in parallel circuit, as shown at L l, Fig. 16, and maintained at a constant potential of 110 volts.

M M' are the flow and return mains, the dynamos bridging them across at one end. If the mains were very long, those near to the dynamos would be exhausting the supply, and the lamps at the remote end would not get the full pressure. A system of feeders has been devised so that each lamp, no matter where it may be, shall have approximately the full 110 volts working through it. Fig. 17 shows a long circuit consisting of two branch mains bridged by a large number of lamps, l l, and D D are the dynamos at the central-station. Series of feeders, f f', have to be taken from the dynamo mains and fed direct into the branch mains at various points, d d', b b', c c', in order to distribute the electrical pressure equally.

The ordinary parallel system is undoubtedly suitable for small installations; but when the area to be lighted is extensive, it is impossible to proportion the mains, with a view to economy in the cost of copper, without sacrificing energy wasted in heating the conductors.

In Figs. 16, 17, the lamps are shown in simple parallel; but if two dynamos are connected together, and a main wire is run from each of their two extreme terminals and a third wire from the branch connecting the two machines, we have what is known as the three-wire system, which was invented by Edison in America, and Hopkinson in England, almost simultaneously. Although by using the third wire there is a saving in copper over the parallel plan, the maximum gain is not more than 25 per cent., under the best conditions; when the district to be illuminated is not more than 400 to 600 yards from the central-station, the three-wire system answers well, but as soon as this distance is exceeded the cost of the mains begins to mount up at a most alarming rate. Although there are many Edison installations in the United States on this system and a few on the Continent, it has only been used here in a few instances for factory lighting.

The Santa Radegonda station at Milan is at the present moment the second largest Edison station in Europe. The building, which was formerly a theatre, is well adapted for the work required; the dynamos and engines are fixed in a deep basement, while the boilers are a few feet above the street level, the upper floors being used as stores and testing-rooms. The dynamos, eight in number, are of the old Edison type, with horizontal magnets; seven of these machines are connected to the feeders which supply the mains, and these cover the district to be lighted on the Edison network system. The motive power is furnished by six Armington-Sims, and two Porter-Allen engines, each connected direct to the armature of a dynamo, the speed being maintained at the uniform rate of 350 revolutions per minute, except in the case of the spare engine and dynamo, which is kept turning slowly, ready to be switched on should occasion demand. The starting or cutting-out of circuit of these large machines requires some care. In the first place, to start, it is necessary to insert resistance into the shunt circuit of the dynamo, which is done by a switch; but to throw 150 horse-power into the main circuit would be dangerous to the lamps, so that the current is first sent into a bank of one thousand lamps used as a resistance, and these are cut out step by step; similar care is taken when a machine is stopped. To control the electro-motive force, which varies greatly from time to time, hand regulation is used during the day, with the help of the Edison tell-tale, consisting of two lamps, a red and white one, which light up when the current is high or low; but when the night service comes on, as it may happen that two thousand lamps may be turned out at once, an attendant has to carefully watch the electric regulator, and be ready to insert resistance into the field-magnet circuits by moving a wheel connected by a shaft and bevel-gear to a system of commutators. The principal difficulty to be overcome, in an installation where the current is distributed over a large area, is the regulation of the electro-motive force at the various points, as at Milan; there are no return galvanometer wires, which are now used in both the two and the three-wire Edison systems in the United States. The plan devised by the company’s electrician at Milan is very ingenious, and enables the pressure at the ends of the various feeders to be kept practically the same, although they are of different lengths and sectional area. In the first place, resistance was added to each feeder to equalise the resistance in each conductor; and, in order to provide for the varying amount of current the feeder has to supply, a peculiar form of commutator, having a guillotine-shaped contact-piece, was inserted in the circuit. By moving this, suitable resistance is inserted or cut out, and the attendant, having a series of numbers, has only to set this instrument to the number shown by the ampÈre meter. By far the largest amount of current is drawn off for the lighting of the Scala Theatre, the stage-lighting alone taking more than one thousand lights: if these were all turned on suddenly, the other lights in the district would be dimmed; to obviate this, auxiliary feeders have been run, which are used only when any great increase is expected; commutators similar to those referred to above also regulate these feeders without any special attention. The pressure at any point in the system is by this means easily controlled, and affords an illustration of what is perhaps not the most economical, but is found to be the most practicable, way of maintaining a constant potential in a district where the amount of output of current is suddenly doubled. Fig. 18 is a plan of the network system of conductors laid through a large portion of the city; the conductors are in outward appearance similar to gas-pipes, the current passing through semicircular bars of copper, embedded both for the flow and return in the same iron tube, which is laid underground in a shallow trench. The house-supply is drawn from the mains, and these are connected to the feeders by means of ordinary junction-boxes, which each contain a fusible cut-out. The bridge-boxes allow of expansion of the line, and have connections for testing purposes. The insulation is extremely good, mainly on account of the favourable nature of the ground, which is chiefly gravel; no trouble has been experienced with leakage, nor has the service ever been interrupted. The cut-outs are of an improved Edison form, but have the disadvantage attending all lead plugs where the current is great, in that, to guard against accidental melting due to the heating effect of the current, the sectional area of the lead has to be much larger than would be otherwise necessary. In fact, these cut-outs will protect the cable against a bad short circuit, but nothing else.

In addition to the glow lamps, eighty arc lamps are worked in derivation, two in series; most of these lamps require 45 volts, to which 10 per cent. of idle resistance is added, constituting a total loss of current which is extremely low for a combined arc and incandescent system of lighting. The service commenced in 1882 with a little over one hundred lamps, and at present there are over ten thousand glow lamps, and two hundred arc lamps are in use. At first the new enterprise had to struggle against very great difficulties; not only the technical difficulties of distribution by means of a network of feeders and mains had to be overcome, but also those arising from the prejudices of consumers and the competition of the gas company, who tried to deter consumers from introducing electric light into their houses. One of these means consisted in offering to the private consumers, resident in the district which was threatened by competition with electricity, an agreement by which the gas company bound itself to supply gas at 5s. 8½d. per 1000 cubic feet, instead of 7s. 7d. as charged hitherto; and even now those inside the “charmed circle” of the electric light conductors get their gas cheaper than the public outside. One of the reasons which accelerated the adoption of electric light was the introduction of the Edison meter, in consequence of which consumers could be charged exactly for the amount of light they had received, and were relieved from paying a lump sum according to the number of lamps fixed, which was customary in the early days of the company. The prices at which the company now provides light, at all hours of the day and night, are as under:—

that is, a little over ½d. per ampÈre-hour; the 10-candle lamps requiring 0·5, the 16-candle lamps 0·75, and the 32-candle lamps 1·5 ampÈre.

The company lends meters for 50, 100, and 150 lamps, at an annual rent of 4s. 10d., 7s. 3d., and 9s. 7d. respectively, and replaces, without charge to the consumer, any lamp the filament of which has broken, but it does not replace lamps where the glass is broken. For arc lamps requiring 9 to 10 ampÈres, an annual rent of £2 must be paid for the lamp itself, and a charge of a little over ½d. per hour for every ampÈre-hour. The carbons are charged for at 1d. per pair, lasting for about seven hours. Now that the installation has been in use for several years, and that the company has arrived at a very accurate estimate of the time during which an average consumer requires the light—about one thousand six hundred lamp-hours per annum—it proposes to simplify the method of charging large consumers, by omitting the initial charge of each lamp, and, instead, to charge 0·6d. for each 16-candle lamp-hour.

The Edison meters are based on the electrolytic action of a small fraction of the current which passes through the meter. They are cells, with rectangular zinc plates immersed in a solution of sulphate of zinc of 1·054 density, the distance between the plates being a little over ¼ inch. The proportion of the current which passes through the meter to that which passes directly into the consumer’s house is 1 to 973. The resistance of the shunt circuit is 9·75 ohms, made up as follows: cell, 1·75 ohm; metallic portion, 8 ohms. The resistance of the metallic portion rises with the temperature, whereas that of the cells falls with a rising temperature; and in this manner the small variations of resistance which might take place in the cell are counter-balanced by the equally small variations in the resistance of the metallic portion. A complete meter consists of two similar-sized cells of the same resistance, placed in series. The object of employing two cells is, that when little current is passing, as in the summer months, one cell alone is used, and when the consumption is sufficiently large both cells are employed, and the mean between the two indications is taken as the basis for calculation in number of ampÈre-hours. The quantity of electricity passed through the cell is calculated by the loss of weight which has taken place in the positive plate. An employÉ of the society visits every meter monthly, taking away the old cells and substituting others freshly constructed. A book is kept in which the weights of the new plates and those of the returned plates are entered, and on the basis of these entries the accounts are made up. The largest plates are those in the 100-light meter, and are intended for a maximum current of 75 ampÈres in the main circuit; they are 6 inches long by 2 inches wide. In cases where a larger amount of current is taken, the capacity of the 100-light meter is increased by joining two or more copper strips across the terminals of the cells. The weak point of the system is the removal of the cells, which leaves the adjustment of the account to be paid entirely in the hands of the Electric-Light Company; in spite of this drawback, it is stated that there has not been a single complaint from consumers during the four years in which the meter system has been in use.

It is evident that in so extensive a system of lighting a short circuit now and then between the lamp wires and the earth cannot altogether be avoided. Many of the lamps have been fitted to existing gas fittings, and are beyond the daily supervision of the company’s officers; the faulty place is often not easily accessible, so the first step taken is to discover on which of the two circuits the trouble has occurred. This is done at the station by joining two 16-candle lamps in series across the main conductors and the point of junction between the two lamps is connected to earth by a stout wire. As long as both circuits (positive and negative) are perfectly insulated from earth no current flows through this middle wire, and both lamps remain hardly incandescent; but, if one of the circuits should be in connection with the earth, the lamp which is joined on the other circuit will brighten up, because the potential of the middle wire and that of the faulty circuit are both zero, and consequently the lamp between the middle wire and the sound circuit receives the full pressure of 110 volts. To localise the fault, contact is made between the earth and the sound circuit by means of a fusible plug of known melting point, say for a thirty-lamp supply. If the fault is on a portion of the external circuit, supplying less than thirty lamps, its fusible plug will melt as soon as the sound main is put to earth. If, however, the fault is on a portion supplying more than thirty lamps, the fusible plug which has been inserted at the station between the sound main and the earth will melt instead. A series of fusible plugs are thus tried, increasing in melting capacity until one is found that does not go: in this case, the other plug on the faulty portion has melted, and the consumer’s lamps on that branch are extinguished; the position of the fault is thus localised, and the company proceed to remedy the defect without interfering in the slightest degree with the rest of their system.

The Edison system is also employed at Berlin, in fact the Deutscher Edison Gesellschaft have at the present time a monopoly of the supply of the city from three large central-stations, each of which serves the area in their immediate neighbourhood. The mains differ from those used at Milan in that stranded highly insulated cables, protected with steel wire on the outside, are laid under the pavement in every street throughout the district. With the exception of the Leipziger strasse and Unter den Linden, which are lit with arc lamps suspended from chains running between cast-iron poles 24 ft. high, about 100 to 250 ft. apart, gas is used for the street lighting, and electricity for the interior illumination of many public buildings and private houses; there are also a good many arc lights outside the shops and restaurants. The mains are on the Edison network system, the area of copper being such, that when all the lamps are on there is a loss of energy of 25 per cent.; but this does not occur on an average for more than half an hour a day. No sole concession is given to the company, who simply have the right to take up the pavement and cross streets, and for this permission they are bound to furnish any consumer in the district with a constant supply of electricity at the following charges:—

In addition to this an installation fee of 6s. per lamp is charged, which includes one lamp.

Meters are charged as follows:—

A discount is allowed off this meter charge, varying with the number of hours the light is used in the year.

The cost of gas is about 4s. 9d. per 1,000 cubic feet, so the electric light is slightly the dearer illuminant.

The Aron meter, Fig. 19, is usually employed as the recorder of the electricity consumed. It consists of two pendulums, controlling two distinct clockwork gears. One oscillates at a regular speed, but the other has a permanent magnet, instead of a weight, and is variable in speed. The entire current passes through the solenoid, which is underneath the pendulum, with the magnet; the difference in speed between the standard and variable clocks is given in direct ampÈre-hours by a counter-gearing similar to the index of a gas-meter. An electro-magnet starts each pendulum when the current begins to flow, and immediately it ceases, two detents come into operation and hold the pendulums stationary.